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OOP Interview Questions and Answers 2026 - Top 10 Object Oriented Programming Questio - Admin - 03-21-2026
Object Oriented Programming (OOP) is a core concept tested in technical interviews at every IT company in 2026, especially at companies like Google, Microsoft, Amazon, and Infosys. Whether you're preparing for Software Developer, Java Developer, or Full Stack Engineer roles, these top 10 OOP interview questions are most frequently asked.
Keywords: OOP interview questions 2026, object oriented programming interview, inheritance interview, polymorphism interview, encapsulation interview questions
1. What are the four pillars of OOP?
The four pillars are: Encapsulation (bundling data and methods together, hiding internal state), Abstraction (showing only essential features, hiding complexity), Inheritance (creating new classes from existing ones, promoting code reuse), and Polymorphism (one interface, multiple implementations). These principles enable modular, maintainable, and scalable code.
2. What is the difference between Abstraction and Encapsulation?
Abstraction hides complexity by showing only relevant features to the user (achieved through abstract classes and interfaces). Encapsulation hides internal data by bundling it with methods and restricting direct access (achieved through access modifiers like private, protected). Abstraction is about design level while Encapsulation is about implementation level.
3. Explain the types of inheritance.
Single Inheritance: one child inherits from one parent. Multilevel Inheritance: chain of inheritance (A->B->C). Hierarchical Inheritance: multiple children inherit from one parent. Multiple Inheritance: one child inherits from multiple parents (supported in C++ but not Java, which uses interfaces instead). Hybrid Inheritance: combination of two or more types.
4. What is polymorphism? Explain compile-time and runtime polymorphism.
Polymorphism means one name, many forms. Compile-time (static) polymorphism is achieved through method overloading (same method name, different parameters) and operator overloading. Runtime (dynamic) polymorphism is achieved through method overriding (subclass provides specific implementation of parent method). Runtime polymorphism uses virtual functions and late binding.
5. What is the difference between an Abstract Class and an Interface?
An abstract class can have both abstract and concrete methods, instance variables, and constructors. A class can extend only one abstract class. An interface has only abstract methods (before Java 8) and constants. A class can implement multiple interfaces. Use abstract classes for related classes sharing common code; use interfaces for unrelated classes sharing behavior.
6. What is method overloading vs method overriding?
Overloading occurs in the same class with the same method name but different parameter lists. It's resolved at compile time. Overriding occurs in a subclass that provides a specific implementation of a parent class method with the same signature. It's resolved at runtime. Overloading is about adding behavior while overriding is about modifying inherited behavior.
7. What are access modifiers in OOP?
Access modifiers control the visibility of class members. Public: accessible from anywhere. Private: accessible only within the same class. Protected: accessible within the same class and subclasses. Default/Package-private: accessible within the same package. Proper use of access modifiers enforces encapsulation and protects data integrity.
8. What is the difference between composition and inheritance?
Inheritance represents an IS-A relationship (Dog IS-A Animal) and creates tight coupling between parent and child. Composition represents a HAS-A relationship (Car HAS-A Engine) and creates loose coupling. Composition is generally preferred over inheritance because it's more flexible, easier to test, and avoids fragile base class problems.
9. What are constructors and destructors?
A constructor is a special method called automatically when an object is created. It initializes the object's state. Types include default constructor, parameterized constructor, and copy constructor. A destructor is called when an object is destroyed, used to release resources. In Java, garbage collection handles memory, while C++ requires explicit destructors.
10. What are SOLID principles in OOP?
SOLID stands for: Single Responsibility (a class should have one reason to change), Open/Closed (open for extension, closed for modification), Liskov Substitution (subtypes must be substitutable for base types), Interface Segregation (prefer specific interfaces over general ones), and Dependency Inversion (depend on abstractions, not concretions). These principles improve code quality and maintainability.